Such apparatus, for example percussion boring machines, usually include an automatic striking piston which is caused to reciprocate in the interior of a housing by means of a fluid, for example compressed air, and which imparts its kinetic energy completely to the housing either directly or indirectly through a striking tip which is axially movable in the housing, and possibly also to the ground. In this way the apparatus performs both breaking-up and also displacement and driving work.
Boring machines are also known which not only displace the ground but also excavate it and, for example, convey it away counter to the direction of advance.
Finally, there are boring machines which permit not only straight-ahead boring but also curved boring. Such machines, for example, have a drilling head which is provided with a steering or oblique face which during straight-ahead boring rotates substantially constantly about the axis of the machine but in the case of boring along a curve is at least temporarily non-rotatable. To make this possible the machine may be provided with a drill string or linkage which is connected to a rotary and feed drive.
To facilitate the advance of the machine and/or the conveying away of the spoil it is also known to provide the machine in the region of the drilling head with nozzles which are supplied through a drill string or linkage with a flushing liquid, usually a suspension of bentonite. However, as in the case of the boring machine according to European application 0 195 559, the nozzles may also be supplied with a liquid at such a high pressure that a cutting jet is formed for excavating the ground in the region of the drilling head.
The machines which hydraulically excavate and/or convey away the soil and the machines operating with a steering jet are connected via a hosepipe or a drill string or linkage to an external pump which produces the pressure required in the particular case. The pressures required vary widely: they range from a few bar in the case of drilling with a flushing liquid to over 100 bar in the case of drilling with a cutting jet. Pressures greater than 100 bar are necessary in particular in the case of hard ground conditions. The pumps, which furthermore are subjected to heavy wear when the liquid used is a suspension of bentonite, are correspondingly expensive.